According to the above given formula, If pipe A  and B fills a tank in x and y hours respectively, but a third pipe C empties the full tank in z hours then the net portion filled in 1 hour when all of them are opened together is $\frac{\mathrm{xyz}}{\mathrm{yz}+\mathrm{xz}-\mathrm{xy}}$ hours.

So,

 $$\begin{gathered} \frac{3\times 4\times 2}{(4\times 2)+(3\times 2)-(3\times 4)} \\ =\frac{24}{2}=12 \mathrm{hours} \end{gathered}$$

Therefore, the tank will be filled in 12 hours when all the pipes are opened simultaneously.

Hence option D is correct